In the pipe molding art, it is known to use mold blocks which move through a molding path commonly referred to as a moving mold tunnel. The mold blocks are fed with molten plastic from a plastic extruder and the molten plastic is shaped to the contour of the mold block in the moving mold tunnel. Provided in the moving mold tunnel is a plastic setting device known in the industry as a cooling plug.
A conventional cooling plug is cooled typically by means of chilled water which is run through coils at the surface of the cooling plug. The internal wall of the plastic pipe may be pulled by vacuum onto the surface of the cooling plug to help in setting the shape of the pipe wall.
The above described cooling plug operation is very efficient as long as there is continuous contact with the pipe wall. However, some pipes are made with internal walls which have wall portions which are not in contact with the cooling plug. These wall portions may for example be coupling portions built directly into the wall of the pipe. Such coupling portions have a greater internal diameter than the main body of the pipe and as such are spaced outwardly away from the cooling plug.
The formation of the main pipe body and the coupling portions in the pipe wall is a continuous process. Therefore, following the formation of a coupling portion is a further wall portion which returns to a diameter which once again contacts the exterior surface of the cooling plug. However, the cooling plug during the time of forming the coupling wall portion drops to an unacceptably low temperature. This is because the chilling of the cooling plug is normally offset by the contact with the heated molten plastic. When the heated molten plastic is not in contact with the cooling plug as is the case during the formation of the coupling portion there is nothing to control the drop in temperature of the cooling plug. When this happens, the pipe wall portion following the coupling portion and coming into contact with the overly chilled cooling plug is cooled too much by the plug. This results in excessive, too rapid hardening and even cracking of the pipe wall. This continues until the temperature of the cooling plug has been brought back up to an acceptable level by further contact with the heated plastic of the pipe wall.
As a result of the above there is an undesirable loss of part of the pipe wall immediately following the coupling portion built into the pipe wall.